In steady-state operation of a charge pump, one often exposes pump switches in the charge pump to voltage stresses. These voltage stresses depend on the design of the charge pump and its output voltage. In general, it is desirable to reduce the maximum voltage stress on the FETs that are typically used as pump switches in a charge pump.
One type of charge pump is a series-parallel charge pump 11, an example of which is shown in FIG. 1. The particular embodiment shown, which is a 1:5 step-down charge pump, exposes three pump switches SW00, SW10, SW14 to a maximum voltage stress of four times the output voltage Vout.
Another type of charge pump is a Dickson charge pump 14, an example of which is shown in FIG. 2. For comparison with the series-parallel charge pump 11 shown in FIG. 1, the particular embodiment shown is also in a 1:5 step-down configuration. The Dickson charge pump 14 features four pump capacitors 20A-20D and five interconnecting pump switches 22A-22E, with the first pump switch 22A accepting an input voltage Vin from a voltage source 12 and the last pump switch 22E providing an output voltage Vout to a load 17. Without loss of generality, the load 17 is modeled as a load resistance RL and load capacitance CL in parallel.
An advantage of the Dickson pump 14 is that during steady-state operation, the maximum voltage stress on any one pump switch 22A-22E is only twice the output voltage Vout, not four times the output voltage Vout as was the case with the series-parallel charge pump 11. As a result, the pump switches 22A-22E can be lower voltage rated switches.
However, although the pump switches 22A-22E in a Dickson charge pump 14 experience only modest voltage stresses during operation in steady-state mode, there is still the problem of transient voltage stress across the pump switches during start-up. Such transient voltage stresses can exceed voltage stresses that occur during steady-state operation. To avoid losing the benefit of the Dickson configuration, the initial charging of the pump capacitors 20A-20D is preferably carried out prior to steady-state operation in a way that avoids imposing excess voltage stress on any pump switch 22A-22E. This problem is addressed by a pre-charge circuit 15A shown in FIG. 2.
The illustrated pre-charge circuit 15A includes stacked resistors R0-R4 connected to the pump capacitors 20A-20D. During a pre-charge interval that begins when the input voltage Vin rises from zero volts to its final voltage value, the stacked resistors R0-R4 pre-charge those pump capacitors 20A-20D. The duration of this pre-charge interval depends on a time constant associated with the resistance of the stacked resistors R0-R4 and the capacitance of the pump capacitors 20A-20D.
If the input voltage Vin is ramped up faster than the time constant associated with the pre-charge circuit 15A then the pump switches 22A-22E may be damaged. To avoid voltage stress on the pump switches 22A-22E during this pre-charge interval, it is useful to provide a disconnection switch SWD that is rated to accommodate the input voltage Vin. The disconnection switch SWD isolates the pump switches 22A-22E during the pre-charge interval. Consequently, during the pre-charge interval, the disconnection switch SWD is opened to isolate the pump switches 22A-22E from the input voltage Vin. Then, when the pump capacitors 20A-20D are charged, the disconnection switch SWD is closed and steady-state operation begins.